1. Field of the Invention
This invention relates to a gas sensor and, more particularly, to a gas sensor capable of being utilized as an NOx sensor for sensing the concentration of nitrogen oxides in combustion gas. The principle of the present invention is widely applicable to the sensing of gases other than nitrogen oxides as well.
2. Description of the Prior Art
Emissions of NOx from internal combustion engines used mainly in automotive vehicles and from the combustion equipment of thermal power stations and plants are a cause of photochemical smog and acid rain, are harmful to the human respiratory system and represent a major source of global environmental pollution. For these reasons the detection of noxious gases such as NOx is a major concern and a gas sensor that contributes to a reduction in the size and cost of measurement equipment and that is usable in a variety of environments has been sought.
In recent years much attention has been focused on all solid-state NOx sensors inserted directly into the exhaust gas of an automotive vehicle to sense the gases continuously, and results of related research have been reported. By way of example, a current-type sensor has been reported (see SAE Technical Paper 960334) as a sensor capable of sensing NOx concentration in the high-temperature exhaust gas of an automotive vehicle. This sensor has an ion conductor provided with two chambers. In the first chamber the concentration of oxygen in the measurement environment is made substantially zero and NO2 is reduced to NO by an oxygen pump. A voltage is applied to electrodes provided in the second chamber to ionize the NO produced by reduction of the NO2 as well as oxygen produced by the reduction of NO2 to NO in the measurement environment. The resulting current is then detected to sense the concentration of NOx. Since an oxygen pump that reduces NO2 to NO in order to detect the NOx concentration is employed in this sensor, the sensed NOx concentration varies greatly depending upon the performance of the oxygen pump and the concentration of residual oxygen and the individual concentrations of NO and NO2 cannot be measured.
A semiconductor-type sensor utilizing the semiconductor properties of various oxides and exhibiting a varying electrical conductivity has been reported as an example of an NOx sensor. For example, Japanese Patent Laid-Open Publication NO. 6-160324 discloses a sensor using tin oxide as a gas-responsive element. However, since the sensitivity to NO gas and the sensitivity to NO2 gas differ in this sensor, the concentration of NOx in a measurement environment in which NO and NOx coexist cannot be sensed.
Another arrangement is disclosed by the disclosures of Japanese Patent Laid-Open Publication No. 4-142455. This specification proposes a mixed potential-type NOx sensor having a sensing electrode and a reference electrode disposed in an ion conductor, wherein a potential difference across the electrodes is measured in gas undergoing measurement. Though this sensor is sensitive to both NO and NO2, the sensitivities to NO and NO2 are of opposite polarity. Consequently, outputs indicative of NO and NO2 cancel each other out in a gas in which NO and NO2 coexist and the concentration of NOx cannot be detected accurately as result. In other words, the overall NOx concentration cannot be sensed. In attempt to solve this problem, the inventors have proposed an overall NOx sensor (see the specifications of Japanese Patent Application Nos. 8-85419and 8-165105) in which a solid electrolyte of zirconia is provided with an internal cavity communicating with the measurement environment and NOx is separated into an NO or NO2 gas component, which is then detected. This is a mixed potential-type overall NOx sensor in which one or two chambers are formed in the solid electrolyte of zirconia and NOx (the main components of which are NO and NO2) is reduced to NO or oxidized to NO2 by an electrochemical oxygen pump or by a catalyst in at least one of the chambers, after which the separated NOx component is sensed. However, it is obvious that in a case where the NOx is not completely separated into one of the gas components, NO and NO2 will mix and interfere with each other. For example, if the performance of a catalytic converter for separating NOx into individual gas components deteriorates, this deterioration will cause a direct fluctuation in the sensor output.
Thus, in the gas sensors proposed heretofore, the oxidation of NO or the reduction of NO2 is brought about using means such as a catalyst or an oxygen pump in order to sense the NOx concentration in a measurement environment. Either NO or NO2 is sensed and adopted as the overall NOx concentration after the NOx contained in the environment is changed to either NO or NO2. As a result, the concentrations of each of NO and NO2 cannot be measured. In addition, the sensed concentration of NOx is highly dependent upon the performance of the catalyst or oxygen pump and it is difficult to detect the NOx concentration accurately.
Further, the mixed potential-type nitrogen oxide sensors measure the electrode potentials of the coexisting oxygen and nitrogen oxide gases using activated electrodes for oxygen and for one or both of NO and NO2. However, the equilibrium potential of NO is negative and that of NO2 is positive with respect to the electrode potential solely of the oxygen (air) electrode of the identical electrodes. Consequently, in a case where it is attempted to detect the sum of NO and NO2 as NOx by identical electrodes, the mixed potential of the sensing electrode varies in the negative direction in conformity with the concentration of NO and in the positive direction in conformity with the concentration of NO2. Accordingly, if NO and NO2 coexist simultaneously, the changes in potential cancel each other out and it is difficult to reflect the concentration of NOx. Moreover, since the total change in potential becomes small, the sensor is readily susceptible to noise.
Thus, a mixed potential-type NOx sensor according to the prior art requires a highly active electrode material for the equilibrium of the sensing electrode, and the output signal is not always satisfactory. A mixed potential-type total NOx sensor also is disadvantageous in that a decline in NOx conversion performance has a direct adverse effect upon sensor output.
Accordingly, an object of the present invention is to provide a gas sensor useful for sensing NOx, wherein the sensor produces a stabilized output regardless of the state in which the nitrogen oxides exist and is not readily affected by variations in NOx conversion performance.
Another object of the present invention is to provide a sensor capable of sensing the concentrations of at least NO2 and NO in a measurement environment so that the concentration of NOx can be sensed accurately.
A further object of the present invention is to provide a sensor in which variations in electrode potential caused by NO and NO2 existing in a measurement gas are caused to occur in the same direction so that total NOx can be detected and the sensitivity of the sensor improved.
According to the present invention, the foregoing objects are attained by providing a gas sensor comprising: a solid electrolytic substrate; and at least two electrodes fixed to the substrate; wherein at least a first electrode of the electrodes is disposed in a gas environment to be detected, at least a second electrode of the electrodes is polarized by applying a bias current or bias voltage, and a change in potential of the polarized electrode is measured to thereby measure a gas component concentration in the gas to be detected.
The electrodes are provided with the functions of a sensing electrode, reference electrode and auxiliary electrode, but a single electrode may be provided with two or three of these functions.
In another aspect of the invention, the foregoing objects are attained by providing a nitrogen oxide sensor comprising: an ion-conductive solid electrolyte; an oxygen pumping electrode fixed to the solid electrolyte and active with respect to oxygen; an oxygen sensing electrode fixed to the solid electrolyte and active with respect to oxygen; an oxygen pumping electrode for measurement fixed to the solid electrolyte and active with respect to oxygen and NOx gas; and a reference electrode fixed to the solid electrolyte; wherein the oxygen pumping electrode and the oxygen sensing electrode are formed in a first chamber, the oxygen pumping electrode for measurement is formed in a second chamber, gas diffusing holes are provided between a measurement environment and the first chamber and between the first chamber and the second chamber, the concentration of a gas undergoing measurement supplied to the first and second chambers is held constant by controlling the oxygen pumping electrode by an electromotive force across the oxygen sensing electrode, a constant current is passed across the oxygen pumping electrode for measurement in the second chamber, and NOx concentration is detected by sensing a change in potential, across the reference electrode and oxygen pumping electrode for measurement, that is based upon a change in NO concentration and NO2 concentration.
In accordance with the construction of the present invention, NOx concentrations in a measurement gas need not necessarily be converted to NO or NO2 concentration. The NO and NO2 concentrations are detected by the potential across the oxygen pumping electrode for measurement and the reference electrode, as a result of which NOx concentration can be measured. Furthermore, in a case where the potential responses of the oxygen pumping electrode for measurement and reference electrode with respect to the concentrations of NO and NO2 differ, means for sensing NO2 or NO concentration by a multi-purpose electrode is provided, whereby NO or NO2 concentration in the measurement gas is detected. This makes it possible to measure NOx concentration.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.